The presence of corrosive products, contaminants, metallic particles, oxidation, etc., in fluids such as lubricants can cause problems. For example, contaminants in lubricants can lead to damage of machinery in which the lubricant is utilized, causing unnecessary or accelerated wear on the lubricated members.
Various approaches have been developed to detect conditions such as deterioration or contaminants in fluids. One conventional system described in U.S. Pat. No. 4,646,070 utilizes a pair of capacitor electrodes positioned in a fluid. The fluid serves as a dielectric between the electrodes to develop an AC voltage response across the capacitor electrodes. Based on the response, the deterioration of the fluid is determined. This solution suffers from a drawback because the sensor is large and bulky and is difficult to move from machine to machine to make fluid contamination measurements.
U.S. Pat. No. 5,262,732 describes a system which utilizes an oscillator circuit coupled to a capacitive sensor. The fluid under test is placed in a reservoir containing the capacitive sensor and the oscillator circuit generates a signal having a frequency that increases or decreases depending on the capacitance of the sensor. The system of U.S. Pat. No. 5,262,732 is also rather large and cumbersome and does not lend itself to portability. In the field, it would be difficult to transport the device from machine to machine to analyze the lubricant at the location of the machinery, for example.
Another type of portable sensor is based on impedance spectroscopy. An impedance spectroscopy system includes capacitor electrodes which are modeled as a complex impedance having an effective capacitor in parallel with an effective resistor. The fluid sample is placed over the capacitor electrodes and forms the dielectric therebetween. The magnitude and phase of the system impedance are then measured at a plurality of different frequencies, and by analyzing the magnitude and phase of the impedance over the frequency range, changes in the permittivity and conductivity of the fluid sample are identified and used to identify the type and level of fluid contamination. The manner in which the complex impedance (magnitude and phase) is used over a range of frequencies to identify various types of fluid contamination can be seen in prior art FIGS. 1a and 1b.
For example, at low frequencies, the reactance of the effective capacitor is large and current will flow primarily through the effective resistor. Thus a low frequency impedance reading provides information regarding the conductivity of the fluid and thus can be used to provide information about the presence of water as a contaminant in the fluid sample. At higher frequencies, the reactance of the effective capacitor is less than at low frequencies and more current will thus flow through the effective capacitor. Therefore at higher frequencies, the measured impedance (magnitude and phase) provides more information about the permittivity of the fluid. The high frequency information thus can provide information about the effects of wear debris and oxidation on the fluid sample.
Metallic particulate contaminants from machine wear and lubricant deterioration due to oxidation may be detected more readily through a higher range of frequencies by indirectly evaluating the permittivity of the fluid because the permittivity of a fluid describes how the fluid responds in an electric field. For example, when a fluid breaks down, experiencing oxidation, it is hypothesized that the hydrocarbon chains in the molecular structure split and form polar dipoles. Therefore, under the influence of an electric field, the dipoles will become oriented in the direction of the electric field. When the electric field changes its direction, the dipoles also re-orient themselves with the changing field. As the frequency escalates, it becomes more difficult for the dipoles to keep pace with the field variations. The maximum frequency at which the dipoles can keep up with the changing electric fields is called the relaxation frequency. At frequencies greater than the relaxation frequency the dipole alignment with the changing electric field lags, causing a drop off in the phase of the complex impedance. In this manner, one may indirectly evaluate the permittivity of the fluid by measuring the complex impedance of the fluid across the frequency range. Ferromagnetic particulates may also be detected more readily at a higher range of frequencies due to their behavior in electric fields.
Although the above impedance spectroscopy provides a good technique for identifying various forms of fluid contamination, the prior art devices do not provide sufficient sensitivity to reliably identify contamination levels below, for example, about 100 ppm. In view of the foregoing, it is an object of the present invention to improve the fluid contamination sensitivity over the prior art systems and methods.